gear ratio good. tire dia good (& putting a piece of stock across the top of the tire then measureing underneath it down to the ground will get you real close, & will for sure tell you that that part is OK & as you said I'm sure it is). GPS dont lie. all that is left is a misprint in the tooth selection chart. we need 26% more teeth (37.8) so the 38 you mentioned would be very near dead on. Just me I would do rollout while you're at it as it might bump you to 37 or tell you 38 would be dead on. I jack the left side up higher than needed to minimize fluid loss. EDIT tho going to the low side (37) might save you a ticket from an overzealous cop.